1. Field of the Invention
This invention relates to a method of producing, by use of a plaster mold, resin articles such as dental prosthetic goods or industrial trial articles and the products thereof.
2. Prior Art
Heretofore, the step of injecting the resin into a plaster mold during the fabrication of dental plates and other dental prosthetic goods was typically performed as a manual operation, with a relatively slow setting resin material being used in combination with a plaster mold contained in a metal flask. However, the resultant product had poor dimensional stability due to nonuniform pressure polymerization, thus making the manufacture of an article of accurate size impossible. As an alternative to such a manual operation, a conventional injection molding machine could be used with plaster molds. However, the molding process intended for use with plaster molds and that intended for use with metal molds are basically different from each other in that different injection conditions are involved. In general, injection molding is a process wherein plastic material is fed by a screw into a heated cylinder and plasticized by external mechanical energy, from solid to liquid state, charged into the mold by external kinetic energy and again changed into a solid through absorption of the energy by the mold. Thus, the process and the quality of the completed articles are greatly influenced by external molding conditions. As a result of improper injection conditions, many different types of molded products may have defects such as short shots, flashing, flow marks, jetting, weld lines, gas burning, air bubbles, clouding, cold marks, strain, warping, cavities, noncompliance to standards, intershot standard errors and interrod standard errors. As examples of the kinds of injection conditions involved may be enumerated; cylinder heating temperatures for plasticizing the material, screw RPM's, the amount of the material to be stored in the injection cylinder for injection into the mold, metering strokes, screw back pressures, injection pressures for charging and pressurizing into the mold, injection speed, injection time, cooling temperature for absorption of thermal energy, cooling time, etc. These conditions are critical and indispensable for producing quality molded articles. For instance, if the cylinder temperature is too high, the properties of the plastic material are lost through deterioration of the material. If the metering stroke is excessive, the resin will stay in the cylinder too long, resulting in deterioration of the material. If the injection energy is too low, the lustre proper to the resin is lost, sometimes causing cold marks. Moreover, if the cooling time is short, the molded product may be unstable in shape and warped. Thus, skilled operators were required to operate the machine, since the molded products were checked by inspection by the operators, who then had to make appropriate adjustments to the molding conditions. The molding conditions are interrelated to one another and quality defects in the molded product cannot be adjusted out unless it is known how the plastic material is affected under various conditions throughout the entire process, and what is happening in the mold.
As the molding conditions are not constant, but are changing at all times to a more or less degree, the product quality per each shot cannot be maintained easily. As a practical matter, the physical properties of the plastic material are subject to the effects of changes in hydraulic pressure, oil temperature, mold temperature and heating temperature. The molding machine must remain stable under the effect of these changes. Fluctuations or discrepancies in the product quality even in the absense of such changes may be attributable to variations in plasticizing factors, including: air discharge from the resin, material purity, mixing of regenerated articles, mixing of compound material, and viscosity of regenerated articles. These factors have been confirmed experimentally to account for a majority of fluctuations occurring in the products and these factors are extremely difficult to be placed under effective quality control.
From the above considerations, it follows that the proper amount of evenly plasticized molten resin must be accurately charged into the mold with an injection energy appropriate to the mold, and that the proper injection conditions must be accurately maintained until the sprue runner and/or gate has been sealed and the product has been cooled for the proper cooling time to the proper cooling temperature. The problem is then how to control these factors. The status functions for the plastic material comprise three parameters: resin pressure, resin capacity and resin temperature. It has been proposed to provide a sensor in the mold for sensing the resin temperature and feeding the sensed temperature back to a heating means mounted at the entrance to the gate. However, an expedient, as a practical matter, has too slow a response time, in view of the high injection speed of the resin which may be of the order of much less than one second. The resin capacity can be made constant by maintaining the mold clamping state constant. Changes in the charged quantity of resin with which the cavity is filled can be sensed indirectly through pressure increase in the resin.
Referring to FIG. 8 (which graphically illustrates a typical relationship between the mold inside pressure and time) such defects as flow marks, jetting weld lines, gas burning, bubbles, cloud, cold marks, strain and warping occur during charging (time point A in the drawing); such defects as flashing, or strain occur immediately before completion of charging (time point B); such defects as short shot, flashing, warping, cavity nonconformance to standards and intershot standard errors occur during pressure holding (time point C) and such defects as warping occurs during cooling (time point D). Thus, a majority of defects are caused during charging. Accordingly, the most important conditions for the molding of the resin is pressure.
The factors discussed above are also relevant to the molding of one-of-a-kind dental prosthetic articles, such as denture plates. However, such articles typically are complex and varying in shape, and comprise many intricate thick and thin portions; their shape is frequently such that undercuts are unavoidable in the one or two piece plaster molds normally employed, and therefore the mold must be destroyed after a single use. Accordingly, when fabricating such articles, it is not possible to make a series of test shots for the purpose of adjusting the machinery to fine tune the molding conditions, nor is it possible to make minor changes to the mold itself, such as varying size or location of a sprue or vent.
Furthermore, plaster molds are not as sturdy as metal molds, and accordingly cannot withstand the pressure extremes normally associated with modern high-speed injection methods utilized for the mass production of articles from high strength polycarbonates and other fast-setting resin materials. This is especially important in the manufacture of resin articles having both thick and thin portions, where it is desireable to use high injection speeds and high injection pressures to force the resin into the thin portion of the molds before it starts to set. Thus, even the best prior art denture plates manufactured by means of plaster molds had a minimum thickness of 1.5 mm, which is excessively thick from a physiological point of view.
Hence, it has long been desired to provide a method for the molding of resin under optimum conditions into a plaster mold, wherein the correlation among the injection speed, inside pressure of the mold and the holding pressure on the one hand and the sprue runner and gate dimensions with air vent dimensions on the other hand may be specified and controlled by appropriate technical measures, so as to result in the convenient fabrication of one-of-a-kind molded articles such as dental prosthetic goods having the desired physical properties and dimensional stability that are associated with modern resin materials.